1,720,984 research outputs found
Filling the gap between transient and steady shear rheology of aqueous graphene oxide dispersions
Full text linkEven though the rheological behavior of aqueous graphene oxide (G-O) dispersions has been shown to be strongly time-dependent, only few transient measurements have been reported in the literature. In this work, we attempt to fill the gap between transient and steady shear rheological characterizations of aqueous G-O dispersions in the concentration range of 0.004 < ϕ < 3.5 wt%, by conducting comprehensive rheological measurements, including oscillatory shear flow, transient shear flow, and steady shear flow. Steady shear measurements have been performed after the evaluation of transient properties of the G-O dispersions, to assure steady-state conditions. We identify the critical concentration ϕ c = 0.08 wt% (where G-O sheets start to interact) from oscillatory shear experiments. We find that the rheology of G-O dispersions strongly depends on the G-O concentration ϕ. Transient measurements of shear viscosity and first normal stress difference suggest that G-O dispersions behave like nematic polymeric liquid crystals at ϕ/ϕ c = 25, in agreement with other work reported in the literature. G-O dispersions also display a transition from negative to positive values of the first normal stress difference with increasing shear rates. Experimental findings of aqueous graphene oxide dispersions are compared and discussed with models and experiments reported for nematic polymeric liquid crystals, laponite, and organoclay dispersions
Structure-directing effect of single crystal graphene film on polymer carbonization and graphitization
No full textWe report the effect of single crystal graphene on carbonization and graphitization of thin film polymers. Electron microscopy reveals that at the early stages of carbonization, graphene induces the carbon atoms in its near vicinity to form oriented layers parallel to the graphene layer. At elevated temperatures, these layers develop further to form extended graphitic (002) planes parallel to the graphene surface. For the samples which were heat treated and graphitized, grazing incidence X-ray scattering reveals that graphene narrows the distribution of graphite grain orientations. Based on these results, we propose that graphene can act as a structure directing agent in both the carbonization and graphitization of polymer thin films, and may provide an approach to realize single crystal graphite films, perhaps in combination with established techniques such as stress recrystallization
Efficient Metal-Free Electrocatalysts from N-Doped Carbon Nanomaterials: Mono-Doping and Co-Doping
No full textN???doped carbon nanomaterials have rapidly grown as the most important metal???free catalysts in a wide range of chemical and electrochemical reactions. This current report summarizes the latest advances in N???doped carbon electrocatalysts prepared by N mono???doping and co???doping with other heteroatoms. The structure???performance relationship of these materials is subsequently rationalized and perspectives on developing more efficient and sustainable electrocatalysts from carbon nanomaterials are also suggested
Graphene Coatings as Barrier Layers to Prevent the Water-Induced Corrosion of Silicate Glass
No full textCorrosion-protective coatings for silicate glass based on the transfer of one or two layers of graphene grown on copper by chemical vapor deposition have been demonstrated. The effectiveness of graphene to act as a glass corrosion inhibitor was evaluated by water immersion testing. After 120 days of immersion in water, bare glass samples had a significant increase in surface roughness and defects, which resulted in a marked reduction in fracture strength. In contrast, the single- and double-layer graphene-coated glasses experienced negligible changes in both fracture strength and surface roughness. The anticorrosion mechanism was also studied.close0
3D microprinting of inorganic porous materials by chemical linking-induced solidification of nanocrystals
No full textAbstract Three-dimensional (3D) microprinting is considered a next-generation manufacturing process for the production of microscale components; however, the narrow range of suitable materials, which include mainly polymers, is a critical issue that limits the application of this process to functional inorganic materials. Herein, we develop a generalised microscale 3D printing method for the production of purely inorganic nanocrystal-based porous materials. Our process is designed to solidify all-inorganic nanocrystals via immediate dispersibility control and surface linking-induced interconnection in the nonsolvent linker bath and thereby creates multibranched gel networks. The process works with various inorganic materials, including metals, semiconductors, magnets, oxides, and multi-materials, not requiring organic binders or stereolithographic equipment. Filaments with a diameter of sub-10 μm are printed into designed complex 3D microarchitectures, which exhibit full nanocrystal functionality and high specific surface areas as well as hierarchical porous structures. This approach provides the platform technology for designing functional inorganics-based porous materials
Folding Large Graphene-on-Polymer Films Yields Laminated Composites with Enhanced Mechanical Performance
Full text linkA folding technique is reported to incorporate large-area monolayer graphene films in polymer composites for mechanical reinforcement. Compared with the classic stacking method, the folding strategy results in further stiffening, strengthening, and toughening of the composite. By using a water-air-interface-facilitated procedure, an A5-size 400 nm thin polycarbonate (PC) film is folded in half 10 times to a approximate to 0.4 mm thick material (1024 layers). A large PC/graphene film is also folded by the same process, resulting in a composite with graphene distributed uniformly. A three-point bending test is performed to study the mechanical performance of the composites. With a low volume fraction of graphene (0.085%), the Young's modulus, strength, and toughness modulus are enhanced in the folded composite by an average of 73.5%, 73.2%, and 59.1%, respectively, versus the pristine stacked polymer films, or 40.2%, 38.5%, and 37.3% versus the folded polymer film, proving a remarkable mechanical reinforcement from the combined folding and reinforcement of graphene. These results are rationalized with combined theoretical and computational analyses, which also allow the synergistic behavior between the reinforcement and folding to be quantified. The folding approach could be extended/applied to other 2D nanomaterials to design and make macroscale laminated composites with enhanced mechanical properties
Ultrahigh Strength and Modulus Graphene-Based Hybrid Carbons with AB-Stacked and Turbostratic Structures
No full textGraphene-based hybrid carbons composed of a mix of AB-stacked and turbostratic regions are reported. Macroscopic graphene films consisting of stacked graphenes are prepared using a liquid crystal graphene oxide dispersion. The graphene films are then infiltrated with bioinspired adhesives, catecholamines, and polymerized to obtain graphene/poly(catecholamine) composites. After heat treatment up to 3000 oC, the composite films are transformed to have both AB-stacked (mainly from graphene oxide) and turbostratic (mainly from poly(catecholamines)) structures, and exhibit significantly improved mechanical properties compared to the films having a predominant AB-stacked structure made from only graphene oxide. They have almost twice the fracture strength (1012 +/- 146 MPa) and approximate to 1.5x increase of both Young's modulus (21.87 +/- 2.24 GPa) and strain-to-failure (8.91 +/- 0.50%). In addition, the films have an in-plane electrical conductivity as high as 1320 +/- 159 S cm(-1). Such hybrid-carbon films with the indicated mechanical and electrical properties have many promising uses, such as for light-weight structural materials, and in flexible electronics such as for wearable heaters or in sensing electrodes
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Dissolving Diamond: Kinetics of the Dissolution of (100) and (110) Single Crystals in Nickel and Cobalt Films
No full textWe report a study of the kinetics of dissolution of (100) and (110) single-crystal diamond plates (???D(100)??? and ???D(110)???) in thin films of nickel (Ni) and cobalt (Co). This dissolution occurs at the metal???D(100) or metal???D(110) interface and was studied in the presence and also in the absence of water vapor at temperatures near 1000 ??C. The single-crystal D(100) dissolves in Ni, and also in Co, in the temperature range 900???1050 ??C. The dissolution is too slow to measure below 900 ??C. In an argon (Ar) atmosphere (under an Ar(g) flow at 1000 sccm and 1 atm pressure, with no water vapor present in the reaction chamber) and at any temperature in the range 900???1050 ??C, the metal film is rapidly saturated with dissolved carbon (C), thin graphite films form on the free metal surface and at the metal???D interface during heating at or above 650 ??C, and the dissolution of the diamond then stops. For addition of water vapor, its partial pressure was controlled by using a water bubbler immersed in a constant temperature bath and Ar(g) was used as the carrier gas. We discovered two different regimes (I and II) for the kinetics of dissolution of D(100) and D(110), in which the rate-determining step was the removal of carbon atoms on the open metal surface (regime I, lower partial pressure of water vapor) or dissolution of diamond at the metal???diamond interface (regime II, higher partial pressure of water vapor) that yielded different Arrhenius parameters. Time-of-flight-secondary ion mass spectrometry depth profiles show the concentration gradient of C from a certain depth into the metal film surface down to the M???D(100) interface, and residual gas analyzer measurements show that the gas products formed in the presence of water vapor on the metal surface are CO and H2. It was found that the rate of dissolution of diamond in Co was higher than that in Ni for both D(100) and D(110) and for both regimes I and II, and possible reasons are suggested. We also found that D(111) could not be dissolved at the Ni/D(111) and Co/D(111) interface in the presence of water vapor (over the same range of sample temperatures). The reaction paths for dissolution of C at the M???D(100) or M???D(110) interface and for removal of C from the free surfaces of Ni and Co were assessed through density functional theory modeling at 1273 K
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